The use of mobile communications networks has increased over the last decade. Operators of these mobile communications networks have increased the number of base transceiver stations in order to meet an increased demand for service by users of the mobile communications networks. The operators of the mobile communications network wish to purchase components for the base transceiver stations at a lower price and also wish to reduce the running costs of the base station. Active antenna systems have proven to meet these goals.
The term “base transceiver station (BTS)” in the context of this disclosure includes, but is not limited to, base stations, as known from GSM networks, as well as a node B (known from UMTS/3G networks) or enhanced node B, and similar units used in other mobile communication network.
The term “subscriber device” in the context of this disclosure is intended to encompass all types of mobile stations and other devices connected to the mobile communication network. Such subscriber devices can be portable or stationary. For example wireless modules can be incorporated into vending machines for the transceiving of data over the mobile communication network. Such wireless modules are also considered to be subscriber devices.
An active antenna system is known, for example, from the Applicant's co-pending international application PCT/EP2007/006334 and U.S. patent application Ser. No. 12/339,239, which is published as US Patent Application Publication No. US 2009/0252205. In the known active antenna system, the antenna is formed by a plurality of antenna elements. At least one of the antenna elements is directly connected to a dedicated radio unit. However, two or more of the antenna elements may be coupled to a single radio unit, for example by a Wilkinson coupler. As the digital radio unit is formed by a plurality of the radio units which are placed close to the antenna elements no coaxial cable is required in order to couple the digital radio unit to the antenna elements, especially if the radio units are arranged on the same printed circuit board or even monolithically integrated by micro-electronic means or as a multi-chip module.
The radio units comprise at least one filter to separate downlink signals and uplink signals, e.g. transmit signals and receive signals. A receive signal is converted into a digital band-pass signal by a low-noise analogue-to-digital converter. The digital band-pass signal is down-converted into a digital base-band signal by a digital down converter, which is integrated in a digital transceiver. In order to generate the transmit signal a digital base-band signal is up-converted by a digital up-converter and transferred by a digital-analogue converter into a RF signal. Each one of the radio units is connected via a so-called C-Hub through an optical fibre to a base station. The C-hub converts the transmit signals and receive baseband signals into optical signals for transmission along the optical fibre according to a CPRI or OBSAI (or similar) standard interface. These optical signals are then transferred via the optical fibre to a digital radio server located at the base station. The optical fibre thereby may be up to 40 km long, enabling the remote location of the base station with respect to the antenna elements mounted on a tower-top.
The radio units, the C-hub and the antenna elements are arranged in one common housing. The common housing is usually mounted on the tower top of an antenna mast and is therefore part of the so-called tower-top equipment. Typically the active antenna system may comprise sixteen antenna elements, but this number is not limiting. The active antenna system with sixteen antenna elements may weigh up to 40 kg.
As the active antenna system contains active circuit elements, the risk of failure is higher than for purely passive antennas. The mobile communications network operators are generally hostile to the idea of having to replace the entire active antenna system in the event of a failure, even if the mobile communications operators can schedule and/or delay this replacement until a number of the modules have failed. The expense and time involved in a crane-based replacement of the active antenna system at a cell site may be a disincentive to the adoption of active antenna systems.